Collaborative Research: Investigation of Anion Incompatibility in the Ca10(PO4)6(OH,F,Cl)2 Apatite Atomic Arrangement

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Apatite, Ca10(PO4)6(OH,F,Cl)2, is the tenth most abundant mineral on Earth and one with fundamental importance in geology, materials science, medicine, dentistry, pollutant mitigation, and as the foundation of the Earth's phosphorus cycle; indeed, all hard tissue of the human body except small parts of the inner ear are made of apatite. All of these applications of apatite require understanding of the atomic arrangement of the mineral. Despite the extensive multidisciplinary literature on apatite crystal chemistry, the atomic arrangements of members of the (OH,F,Cl) binary and ternary systems are not well understood and our current knowledge is full of inconsistencies that must be resolved; apatite is one of the rare minerals for which the atomic arrangement of the solid solutions cannot be predicted from the end-member arrangements.Mixing of the end-member atomic arrangements suggests, for example, that binary members of the system must undergo symmetry breaking, possess immiscibility gaps, incorporate essential vacancies with an unknown method of charge balance, and/or possess anion positions that are not currently recognized. This multi-faceted proposed study will couple mineral synthesis and detailed compositional characterization with single-crystal X-ray structure analysis and Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopic studies in order to elucidate the nature of solid solution among and between the OH, F, Cl ternary apatites, and provide a better fundamental understanding of anionic substitution and phase behavior. The results of this study will have applications in the fields of geology, materials science, medicine, and dentistry. A proof-of-concept study demonstrates that miscibility along the F-Cl join is achieved by the creation of at least four anion sites in the F-Cl anion column, and confirms the efficacy of the proposed synthesis and analysis methods.